Theoretical Research on Sensitivity for Resonance Wavelength of Long Period Fiber Grating

In a long period fiber grating(LPFG), the forward-propagating guided mode in the core is coupled to a number of co-propagating lossy cladding modes at resonance wavelengths. Sensitivity of resonance wavelength against temperature, axial strain, and external refractive index is modeled in this paper. Schemes for temperature compensation and enhanced wavelength tuning of LPFG are proposed and analyzed. Experiments show accordant results with simulations. Temperature coefficient of the LPFG compensated by means of axial strain is below 1.8 nm/100℃. For the LPFG with polymer recoating layer, the enhanced tuning range is more than 70 nm.     

Bend-insensitive long-period fiber grating sensors

A novel bend-insensitive long-period fiber grating (LPFG) sensor written using focused CO₂ laser pulses is demonstrated, for the first time, to our knowledge. It is found that the central wavelength shift of such a LPFG is only −0.018 nm even for a curvature of 1.1 m⁻¹ at the most bend-insensitive position of the LPFG. Experimental results show that the bend sensitivity of the central wavelength of the LPFG has a periodic distribution along its circular directions. Such a bend-insensitive sensor could be used to solve the problem of cross-sensitivity between bend and other measurands, such as temperature, strain or refractive index, which is an unsolved problem for LPFG sensors in practice. In addition, the bend sensitivity of the LPFG can be adjusted by selecting its circular positions.     

All-fiber intermodal Mach–Zehnder interferometer based on a long-period fiber grating combined with a fiber bitaper

We report a novel all-fiber narrow-bandwidth intermodal Mach–Zehnder interferometer (MZI) based on a long-period fiber grating (LPFG) combined with a fiber bitaper, and the MZI has no special limit for the resonant wavelength of the LPFG. Its responses to temperature and axial strain are studied theoretically and experimentally. Experimental results indicate that the temperature sensitivity is 0.0585 nm/°C within the temperature range from 30 °C to 90 °C and the axial strain sensitivity of 0.00013 nm/με can be neglected. Furthermore, as only the common single-mode fiber (SMF) is required during the fabrication process, the proposed device is cost effective and has good practicability in the optical sensing systems.     

基于长周期光纤光栅的磁场传感方案的实现

长周期光纤光栅是近几年出现的一种新型光纤器件。其耦合机理是前向传输的纤芯基模与前向传输的各阶包层模式之间的耦合 ,这与光纤布喇格光栅明显不同。它对温度、应力、弯曲及外部折射率的变化具有相当高的灵敏度 ,是一种理想的光纤型敏感结构。提出利用长周期光纤光栅谐振波长的移动或移动变化量进行磁场强度的测量 ,并进行了理论推导、仿真实验及分析研究。得出了谐振波长变化量与外界磁场强度的关系模型。分析了设计中应该考虑的因素。     

基于周期性矩形压力槽的可调谐长周期光纤光栅

利用机械线加工技术设计矩形压力槽制作了周期为600 μm,周期数为60的长周期光纤光栅.实验研究了所制作长周期光纤光栅的温度和压力特性,并通过调整光纤和压力槽之间的角度改变长周期光纤光栅的谐振波长.采用此技术制作的长周期光纤光栅最大谐振峰值可达15 dB,通过调整光纤与压力槽之间的角度可使谐振波长变化超过12 nm.     

Optimal design of temperature-insensitive long period fiber gratings for athermal refractive-index sensor

Based on coupled-mode theory of long-period fiber grating (LPFG), a theoretical analysis and simulations for the optimal design of a temperature-insensitive LPFG is presented in order to achieve an athermal condition for sensing the refractive index of the external medium. Effects of the variation of the cladding radius and grating period on the temperature sensitivity of the LPFG are discussed. Both of these parameters are found to be important to control the temperature sensitivity when the thermo-optic coefficients of core and cladding materials are of the same order. Other grating parameters are also optimized in order to achieve a good contrast of the grating period with resonance wavelength in the 1.5 μm region and to sense the external medium refractive index over a wide range. Variation of external medium refractive index from 1.0 to 1.45 results a red-shift in the LPFG resonance wavelength by 86 nm with its temperature sensitivity as low as 0.008 nm/°C over a temperature range of 0–80 °C for this optimally designed LPFG.     

基于长周期光纤光栅和保偏光纤Sagnac环光谱特性的温度与应变测量研究

提出并研制了一种结构简单、成本低廉的温度与应变同时测量系统,其结构是在保偏光纤Sagnac环内接入一个长周期光纤光栅(LPFG)。利用LPFG对保偏光纤Sagnac环的透射光谱进行调制,通过监测谐振峰波长和强度的变化,发现波长随温度和保偏光纤的应变变化,强度随LPFG的应变变化,因此可以实现温度与应变的区分测量,并且可判断出应变的施加位置。实验得到该系统的温度灵敏度为0.181 81 nm·℃-1,LPFG区的应变灵敏度为0.005 283 dB·με-1,保偏光纤Sagnac环区的应变灵敏度为0.015 72 nm·με-1。实验结果表明该方案可行,并具有一定的实用性。     

Widely tunable long-period fiber grating with nm-thick higher refractive index film overlay

By introducing a four-layer step-index waveguide modeling, the characteristics of long-period fiber grating (LPFG) with an nm-thick film overlay, which has higher refractive index than that of fiber cladding are investigated in detail. The influence of both the overlay thickness and refractive index on the tuning ability of LPFG is analyzed. The numerical results demonstrate that spectral response of LPFG is divided into three distinct regions as the overlay deposition increases and the shift of resonant wavelength is drastic in some special thickness range. In conjunction with higher-order cladding mode coupling and fiber cladding etching, the sensitivity of LPFG to the overlay refractive index is enhanced significantly and over 120 nm resonant wavelength tunable range is achieved.     

基于Bitaper-LPFG-Bitaper结构的全光纤Mach-Zehnder干涉仪的温度传感特性

提出并制作出一种基于锥体光纤-长周期光纤光栅-锥体光纤结构的全光纤Mach-Zehnder(M-Z)干涉仪传感器,并对其温度传感特性进行了研究.实验结果表明,固定光纤锥体和长周期光纤光栅的结构,仅改变两个光纤锥体之间的距离,对应不同的M-Z干涉谐振峰呈现出不同的温度传感特性:随着两个光纤锥体之间的距离增加,位于短波长处的谐振峰,传感器的温度灵敏度减小,而位于长波长处的谐振峰,传感器的温度灵敏度增加.当传感器长度为16.5 cm时,在1 680 nm附近的温度灵敏度达到0.102 06 nm/℃.实验结果对于锥体光纤-长周期光纤光栅组合型温度传感器的优化设计具有重要参考价值.     

长周期光纤光栅:原理、制备与应用

较全面介绍了长周期光纤光栅。对比Bragg光纤光栅说明了长周期光纤光栅的特点。对其耦合机理进行了分析 ,阐明了波长选择损耗特性。介绍了长周期光纤光栅的制备方法 ,大致分之为基于非形变的制备方法和基于形变的制备方法。详细阐述其在通信、传感领域的应用 ,尤其是新型应用 :带通滤波、光上下路复用、光纤光源、偏振器件、新型传感等。并针对长周期光纤光栅对温度、应力、外部折射率敏感的特性 ,探讨了调谐性以及温度稳定性的方案     

共振波长对导模共振生物传感器灵敏度的影响

导模共振生物传感器由于具有微型化、免标签、高通量和实时检测等优势被广泛研究。利用严格耦合波理论分析了该生物传感器对样品折射率和厚度的灵敏度随共振波长的变化规律。当样品厚度和共振波长一定时,其折射率灵敏度恒定;随着共振波长的增大,折射率灵敏度显著提高。当样品折射率和共振波长一定时,厚度灵敏度随着厚度的增加而降低,最后趋近于0;当共振波长增加时,厚度灵敏度明显提高,厚度测量范围增大。结果表明:选取较长的共振波长有利于样品的检测和分析。     

Unique temperature sensing characteristics of CO2-laser-notched long-period fiber gratings

Long-period fiber gratings (LPFGs) are written by use of a focused CO₂ laser beam to notch periodically on the surface of optical fibers. Temperature characteristics of the CO₂-laser-notched LPFG are investigated in detailed to develop its sensing applications. It is shown that, for such LPFG, temperature sensitivity of resonant wavelength can be increased by creating deeper notches. The coupling from the fundamental core mode to a cladding mode in the LPFG can be enhanced by increasing the ambient temperature. The polarization-dependent loss (PDL) of the LPFG depends strongly on the ambient temperature. Such unique temperature characteristics can be used to solve the cross-sensitivity problem between the temperature and the tensile strain. So, the LPFG is a promising temperature sensor based on the wavelength modulation, on the intensity modulation and/or on the PDL modulation.     

对弯曲不敏感的长周期光纤光栅传感器

发现了用高频CO₂激光写入的长周期光纤光栅的谐振波长对弯曲灵敏度在圆周的不同方向上呈现周期性,且在两个特定的对称方向上对弯曲不敏感(即弯曲曲率达到1.1m^-1,谐振波长仅漂移-0.018nm).据此提出新型对弯曲不敏感的传感器和可调灵敏度弯曲传感器的设计方案,可望从根本上解决长周期光纤光栅在测量中存在的温度、应变或折射率与弯曲之间的交叉敏感问题.     

基于光纤光栅的可调谐半导体激光器

本文利用光纤光栅(FBG)形成外腔反馈及F-P半导体激光器的多模特性,得到波长稳定、可调谐的半导体激光光源,系统研究了不同反馈比率对激光器光谱特性的影响.通过向FBG施加轴向拉力和压力可实现10nm的波长调谐,得到一系列波长间隔0.94nm的激光输出,边模抑制比达35dB,光谱线宽小于0.2nm.     

低成本长周期光纤光栅传感系统

研制了一种低成本长周期光纤光栅传感系统.利用高频CO2激光脉冲写入长周期光纤光栅边缘滤波效应原理,设计参考光路以补偿进入光电探测器前光路中各种扰动引起的光功率起伏.当输入为单波长信号光时,可以通过测量探测器输出的电信号来获得光纤光栅所承受的温度和应变信息.在温度实验里,温度和探测到的电信号成较好的二次关系,温度分辨率为0.014℃；在应变实验里,施加的应变和探测到的电信号成线性关系,应变分辨率为2.5 με.该传感系统结构简单,成本较低,响应快.     

长周期光纤光栅应变和温度传感灵敏度研究

基于模耦合理论,从长周期光纤光栅在外加轴向应变或环境温度变化时可能出现的弹性形变、弹光效应、热光效应、热膨胀效应及波导效应入手,推导了描述其轴向应变和温度灵敏度的一般性公式.利用数值方法研究了光栅周期、耦合模式的阶次、弹光系数及热光系数对灵敏度的影响.结果表明：在相同条件下,增大光栅周期将是提高灵敏度的有效手段之一;低次模较高次模的耦合具有更高的灵敏度;包层与芯区的弹光系数（热光系数）相等时,波长漂移量很小且与谐振波长值有简单的正比关系.     

长周期光纤光栅谐振波长的横向负载特性分析

在考虑波导效应、弹光效应等因素后,详细地理论分析了长周期光纤光栅(LPFG)的横向负载传感特性,包括谐振波长漂移及谐振峰分化后的双峰间距与横向负载、光偏振的关系,模拟计算了单模光纤长周期光栅的横向负载传感特性.理论分析和计算结果表明,长周期光纤光栅谐振波长随横向负载的漂移量和方向与光源的偏振方向有关,加载方向偏振光的谐...     

基于相移光纤光栅的布拉格波长解调原理的分析

提出了一种基于相移光纤光栅(phase shifted fiber grating)的布拉格波长解调技术，相移光纤光栅在反射谱阻带中能打开线宽极窄的一个或多个通透窗口，而且窗口位置随所加相移量的大小呈线性变化。实验中应用电流调谐相移量以实现布拉格波长的解调。选用最大波长偏移量为2nm的相移光纤光栅进行调谐滤波，温度的测量范围可达到200℃，应变的测量范围可达到2000μs。实验表明利用相移光纤光栅进行布拉格波长解调，取得了预期的效果，为布拉格波长解调技术提出了新的思路。     

监测点波长对高双折射光纤环镜轴向应变灵敏度的影响

研究了高双折射光纤环镜监测点波长对轴向应变灵敏度的影响. 经理论推导得出高双折射光纤环镜轴向应变灵敏度公式. 结果表明: 当高双折射光纤材料选定后, 灵敏度随着监测点波长的增大而增大; 当监测点选定后, 监测点的灵敏度为常数, 波长变化与应变成线性关系. 实验选取不同波长的波峰进行应变灵敏度监测, 对各监测点进行数据拟合, 实验结果与理论分析一致. 研究结果对提高高双折射光纤环镜应变、温度等灵敏度具有指导意义. 关键词： 波长 灵敏度 监测点 高双折射光纤环镜     

基于F-P标准具的固体激光可调线宽控制技术

钠导星激光研制需激光线宽在一定范围内可调节。开展了F-P标准具压窄线宽数值模拟,设计一台高光束质量基横模线偏振激光输出全固态1064 nm激光器作为实验装置,结合激光器输出光谱特性进行计算。根据计算结果采用不同规格的单标准具、组合双标准具实现了激光线宽在0.75 ~2.83 GHz范围内的离散调节控制。在标准具未控温情况下,不同线宽时激光中心波长保持1064.58 nm,30 min内波长漂移量小于3 pm。